Signaling in a telecommunications network is the act of transferring signals or signaling information pertaining to call management and processing. Signaling is simply the information exchanged between switching nodes or signaling transfer points and may be used to perform four basic functions: supervising, alerting, addressing, and billing. Supervising involves monitoring the status of a line or circuit to see if it is busy, idle, or requesting service. In an automated telecommunications network, supervisory signaling information is provided to reflect or direct the status of a line or a circuit such as the on-hook/off-hook status. Alerting or notifying involves providing signaling information indicating the arrival of an incoming call at the end user's telephone device. Addressing involves providing address signaling information indicating the address of a calling party, a called party, or a telecommunications network element. The address signaling information is used for transmitting routing and destination signals over the network. Billing involves providing signaling information to properly bill a call such as by providing information regarding the billing number, the duration of a call, and the level of service.
In most telecommunications applications, the user provides signaling information along with the voice or data signal and the signaling is thus referred to as in-band signaling. In-band signaling is primarily provided using multi-frequency and single frequency signals that are provided along with the voice or data signal in the same circuit. For example, in-band signaling may include the use of tones which pass within the voice frequency band and are carried along the same circuit as the talk path that is being established by the signaling information. In-band signaling suffers several disadvantages such as a reduction in available bandwidth for transmitting information because of the presence of the signaling information. Another disadvantage includes the use of full bandwidth channels to provide signaling information that requires only a fraction of this bandwidth in many situations. In-band signaling also increases the accessability of sensitive information such as billing, addressing, and monitoring information which increases the opportunity for fraud. Further disadvantages of in-band signaling includes relatively slow performance when setting up and disconnecting circuits.
As a result of these and other disadvantages, a completely separate signaling network was developed that is dedicated to providing signaling information separately from the channel or circuit carrying the content such as voice or data information. This may be referred to as out-of-band signaling and has been implemented, primarily, to provide interoffice signaling. Out-of-band signaling increases the overall network bandwidth and allows for more sophisticated telecommunications services to be provided because of the added flexibility of providing content through one network and signaling through another. For example, signaling information may be exchanged over the signaling network between an originating switching system and a destination switching system before a content or traffic circuit is established to determine if the called station is busy. If busy, a content circuit or channel is not established allowing it to be used for another call.
Out-of-band signaling was first implemented using common channel interoffice signaling (CCIS) for transmitting signaling information for a group of trunks over a separate channel. CCIS uses packet switches in its signaling network that may be referred to as signal transfer points (STPs). Today, signaling networks are using a newer out-of-band signaling system and protocol developed by the International Telegraph and Telephone Consultative Committee (CCITT) which is called Signaling System 7 (SS7). SS7 uses an out-of-band signaling protocol that may be implemented in digital signaling networks. SS7 provides a layered functional structure and uses destination routing, octet oriented fields, variable length messages, and a highly reliable message transfer protocol. SS7 also provides flow control, connection and connection-less services, and Integrated Services Digital Network (ISDN) capabilities. Out-of-band signaling solves many of the disadvantages associated with in-band signaling.
A typical out-of-band signaling network will include a local digital switch, transmission facilities with associated transport devices including a first channel bank and a second channel bank, an STP, and a service control point (SCP). The STP may be implemented as a specialized packet switch optimized for SS7 packets. The SCP may be used to control an associated local digital switch, or a tandem switch in other embodiments, that supports intelligent network services. In such a case, the local digital switch may be referred to as a service switching point (SSP). The local digital switch and the first channel bank may be employed at a local central office, and the first channel bank may be coupled to the second channel bank located at a second location through a high-speed transmission facility so that signaling information may be exchanged in both directions. The second channel bank may couple to the STP which then couples to the SCP also located at the second location. The second location may be provided anywhere such as another central office, a tandem switch office, or an inter-exchange carriers tandem switch office. The local digital switch receives line and address signaling information from users through subscriber loops and interfaces such as digital subscriber loops, loop carrier systems, and digital cross-connect switches. The local digital switch then provides the signaling information to the first channel bank where it is sent to the STP through network transmission facilities and the second channel bank. The channel banks serve as transmission systems that condition the signaling information so that it may be transmitted across a high-speed network transmission facilities between two network office locations.
In such a signaling network, signaling information may be exchanged in both directions. For example, signaling information may be received at the local digital switch and provided to the first channel bank. The first channel bank serves as a transmission system to condition the signaling information for transport on a high-speed circuit, such as a T-1 circuit, so that the information may be provided to the second channel bank at the second location. For example, the signaling information may be provided at a digital signal, level zero (DS0) rate and is then combined into a digital signal, level one (DS1) rate for transmission on the high-speed transport facility. The second channel bank also serves as a transmission system and terminates the high-speed transport facility carrying the signaling information and demultiplexes it back into its individual signaling information links to be provided to the STP for routing to the SCP or another SSP. The SCP may be implemented as a computer and associated database that includes network and customer-specific information to perform such tasks as call routing and number (address) translation to deliver network services. As a result of querying the SCP, additional signaling information, such as routing and translation signaling information, may be generated and provided to other STP's from the local STP so that appropriate switching and services may be established and provided.
Telecommunications services, such as services provided by the advanced intelligent network (AIN), are becoming more sophisticated and prevalent. Furthermore, new signaling protocols and standards, such as the telephony application programming interface (TAPI) and the telephony services or server applications programming interface (TSAPI), are becoming more prevalent and available to provide local call processing and control at a user access element such as a private network. Unfortunately, the typical out-of-band interoffice signaling network configuration described above does not allow for signaling information to be easily and conveniently exchanged with non-traditional signaling information end points or user access elements. This prevents enhanced local control and call processing.
Non-traditional signaling information end points are those access elements that cannot or do not normally receive signaling information and thus do not serve as signaling end points. Currently, the local digital switch is the only traditional end point capable of providing and receiving signaling information and signaling traffic in a local central office. The non-traditional signaling information end points or user access elements may include such elements as digital loop carrier equipment, digital cross-connect systems, private telecommunication networks, computer networks, or computers and telecommunications systems implementing such protocols as TAPI and TSAPI. Non-traditional signaling information end points may also include private telecommunications networks and ISDN services with the capability to receive and process signaling information in such formats as Transaction Language 1 (TL1), X.25, and the various Q.9xx/Q.29xx signaling protocols and standards that may be used with ISDN and broadband ISDN (BISDN) services.
The inability to directly exchange signaling information between an interoffice signaling network and a user access element in a desired format prevents the expansion of the network control plane. The signaling information cannot be provided to non-traditional information end points or access elements in a desired format from either the local digital switch or the first channel bank. For a variety of reasons, the local digital switch, which receives the signaling information, cannot be easily modified to provide the signaling information to non-traditional end points or to convert the signaling information from one format to another. Digital switch manufacturers are reluctant to modify existing software to provide such capabilities. This inability to provide signaling information to non-traditional end points or access elements harms the overall public telecommunications network and the development of the more advanced telecommunications services, such as services provided by the advanced intelligent network (AIN). For example, as telecommunications system usage continues to rise, with the increased popularity of such services as Internet access and video services, the capability to use signaling information to route high bandwidth services to an appropriate high bandwidth switch to relieve congestion on existing local digital switches requires the ability to provide signaling information to non-traditional end points or access elements.
In existing signaling networks, the signaling information cannot be provided from the standard channel banks which are essentially provided as "dumb" transmission equipment without the capability or intelligence to convert signaling information from one format to another. The standard channel banks contribute to reduced reliability and are a potential source of link failures. The channel banks are further limited in that they cannot provide network security (firewall) capabilities to help prevent harm to the signaling network by eliminating bogus or fraudulent signaling traffic. The channel banks cannot apply priority services for selected message flows and cannot provide message traffic statistics to support signaling network diagnostics or inter-service provider billing capabilities. Furthermore, the channel banks cannot provide link concentration to carry multiple access link traffic flows on a common link into the interoffice signaling network backbone or SS7 backbone. As such, access elements, such as multiple private networks, can access the signaling network only by providing dedicated, signaling transmission facilities to communicate directly with the STP. These signaling transmission facilities are expensive and frequently must be provided over long distances. Additionally, expensive interface hardware must be provided for each private network accessing the STP.